DE 38 24 710 C2 discloses a device that is suitable for suctioning granular material from the floor in order to clean the material. A blowing air stream branches off from the pressure side of the blower that generates the suction air stream, and is directed onto the granular material, lying on the floor, via a separate channel.
A device is known from DE 33 18 756 C2 for receiving wastes by means of a suction air stream that is impelled by a blower. A portion of the air stream, generated by the blower, that is recirculated in a separate channel is used for removing wastes from the floor.
EP 1 211 354 A2 describes a method for cleaning lightweight, fine-particle contaminants from stone materials. For this purpose, a suction excavator is used which has a suction tube that is connected to a suction pump via a tank. The air volumetric flow rate in the suction tube is set so that the stone materials are kept suspended, while the more lightweight contaminants are suctioned off. The cleaned stones are then set down again by temporarily reducing the suctioning air volumetric flow rate.
In all suction excavators that are available on the market, radial fans generate large, very fast-flowing air streams that entrain a material to be received (also referred to below as suction material), which is situated in the area of the suction opening of a suction hose. However, at temperatures around the freezing point, the fast-flowing suction air stream results in rapid cooling of the components of the suction excavator upstream from the fan in terms of flow. For this reason, the air-conducting components of the suction excavator may freeze at low temperatures. In addition, moist suction material in the suction hose or in the separator, for example, may freeze on. Likewise, the ultrafine filters used in the downstream filter unit may freeze, resulting in a sharp reduction in the suction air flow and thus, in the conveying capacity.
A system is known from FR 2 286 772 that leads the air, which is compressed in a vacuum pump, back to a suction point via a separate channel. In the process, the heat of the recirculated air may also be utilized to thaw frozen material.
DE 10 2010 060 973 A1 discloses a control method for a suction blower of a suction excavator, in which the suction blower is driven by an internal combustion engine via a drive train. The suction blower is a system made up of a first radial fan and at least one additional radial fan. A bypass line is situated between the suction line of the first fan and the exhaust air line of the additional fan. An internally or externally controlled bypass flap, which in the open state allows partial recirculation of the conveyed air stream and which is closed during normal operation, is situated in the bypass line.
Lastly, a suction excavator is known from WO 2015/024558 A1, which in a known manner includes a suction hose with a suction opening for pneumatically receiving suction material by means of a fast-flowing suction air stream. In addition, a separator for separating the suction material from the air stream, as well as a filter unit for cleaning the air stream are provided. A fan generates the suction air stream. The suction side of the fan is connected to the outlet of the filter unit, and the pressure side is connected to an exhaust air channel. The exhaust air channel opens to the surroundings via an exhaust air opening. In addition, a portion of the exhaust air stream may be led via a recirculation channel to internal components of the suction excavator, for example back into the separator, in order to heat the deposited suction material at that location. Likewise, a section of the recirculation channel extends, separately from the suction hose, to the vicinity of the suction opening in order to convey heated exhaust air to that location. For this previously known suction excavator, however, it is still disadvantageous that a separate channel must be led to the suction opening in order to heat the suction material to be received. Since the pressure in the recirculation channel that is generatable by the fan is not high, this channel must be led with a relatively large cross section up to the suction opening, which not only increases the costs of the suction excavator, but also in particular makes handling the suction hose more difficult. In addition, there is a risk that the drawn-in suction material may once again freeze in the suction hose when the ambient temperatures are low.
Proceeding from WO 2015/024558 A1, it is considered to be an object of the present invention to provide a further improved suction excavator in which freezing in the suction hose is avoided or may be reversed without the need for costly heating elements. A further aim is that attaching a separate recirculation channel that extends to the suction opening may be avoided altogether, or that in any case a smaller cross section is required for the recirculation channel. A further object is to provide a method for the improved control of such a suction excavator.